1. Field of the Invention
The present invention generally relates to computer software. More specifically, the present invention relates to a method and system for generating slope grading transitions in CAD drawings.
2. Description of the Related Art
The term computer-aided design (CAD) generally refers to a broad variety of computer-based tools used by architects, engineers, and other construction and design professionals. Civil engineers frequently use CAD applications to compose design drawings for engineering projects. The design drawings typically provide both two-dimensional (2D) and three-dimensional (3D) representations of a construction project integrated with a digital terrain model. For example, the design drawings may provide a 3D model of a roadway, a parking lot, or a retention pond, to name but a few examples.
One common design requirement for these types of civil engineering projects is referred to as site grading. Generally, “grading” is the process of excavating or building-up grading material and is usually associated with construction foundations, roads, parking lots, etc. Accurate CAD modeling of grading surfaces and volumes is an important issue for civil engineering projects because the cost of hauling away or bringing-in grading materials can be a major expense. Further, improperly graded surfaces may lead to premature failure for elements of a civil engineering project. For example, an improperly graded surface for a roadway may lead to the roadway being washed away over time by surface runoff. Thus, accurately representing both the shape and volume of the grading in a CAD drawing is an important design requirement.
Historically, 2D CAD applications were only able to provide crude approximation tools for estimating the grading requirements of 3D volumes. More recently, CAD applications have been made available that can generate 3D models. Such CAD applications often include tools for modeling grading shapes and volumes using 3D solids. To generate a model of a graded area, the tools typically generate surfaces using a set of overlapping polygons. In a number of common scenarios, however, this can lead to an inaccurate 3D geometry for a graded area. Further, because the geometry of the graded area is inaccurate, any calculations of the 3D volume of the graded area tend to be inaccurate as well.
One common scenario where prior art CAD applications fail to develop an accurate grading model arise when the CAD application is used to create a grading model for a grading transition. A grading transition is where the grading criteria for a graded region transition from one set to another. Grading criteria typically specify a required slope for a graded surface that extends for a specified distance from a given feature line. Using this approach, a set of polygons may be generated by connecting perpendicular lines extending from either end of the feature line. With the overlapping set of polygons, a “daylight line” representing the end of the graded surface may be defined. However, with this approach, if the feature line has a bend, then the daylight line usually is not able to generate an accurate 3D geometry of the graded area or an accurate 3D volume for the graded area (particularly where there are multiple grading criteria moving up or down the feature line). This occurs because the grading distance from the feature line may be measured two different ways, depending on which segment of the feature line is used.
Another drawback of this approach is that on the outside bend of the daylight line this leads to a discontinuous shape for the daylight line that has traditionally been connected with an additional line segment. On the inside bend, this approach leads to a jagged appearance for a portion of the daylight line, where representing how material is actually graded requires a smooth transition. Thus, simply using overlapping polygons to generate a grading transition leads to a poor 3D model, limiting the usefulness of fill/excavation calculations generated therefrom.
Other methods for calculating an approximate grading volumes and daylights are available. For example, samples may be taken at a regular interval along the footprint at the given slope/distance criteria. The daylight is formed by connecting the endpoints of the sample lines, and volumes are calculated by estimating the volume between two adjacent samples. Using this approach, however, the daylight line can have a number of undesirable properties. For example, a daylight line calculated using this approach may intersect with itself. Moreover, the accuracy depends on the number of samples taken.
Accordingly, there is a need in the art for improved techniques for representing the shape and volume of grading transitions for civil engineering projects using a CAD application.